This invention relates to a process for the oxidation of butane in the liquid phase whereby valuable oxygenated organic compounds are produced. It is particularly related to a process for the production of aliphatic monocarboxylic acids such as acetic acid by the oxidation of butane in the liquid phase whereby impurities in the acids which adversely effect the permanganate time thereof are removed by a novel process.
Direct oxidation of saturated aliphatic hydrocarbons such as propane, butane, pentane, hexane, heptane or mixtures of the hydrocarbons, with oxygen is well known. In particular, the oxidation of butane is a well-known process which has been described in numerous patents and technical publications relating to both liquid phase and vapor phase oxidation both catalytically and non-catalytically. Liquid-phase catalytic oxidation (LPO) of n-butane was introduced by Celanese in 1952 in Pampa, Tex. The oxidation product during liquid phase oxidation of paraffin hydrocarbons such as butane contains a light ends boiling up to about 99.degree. C., water, aliphatic monocarboxylic acids of 1-4 carbon atoms and high boiling residues. The major low-boiling constituents are ethyl acetate, methyl ethyl ketone and methyl vinyl ketone, along with traces of aldehydes and other esters. The acids which are produced include mainly formic acid and acetic acid with smaller amounts of propionic acid, acrylic acid and butyric acid. A number of carbonyl compounds form the higher substances. To recover the acetic acid and other acids from the oxidate product, a series of distillation steps are utilized. Unfortunately, the acetic acid and other acids produced in such a way have not always passed the various tests for permanganate time. Since a sufficient permanganate time is an important commercial test which the acid product must meet for many uses, the presence therein of impurities which degrade permanganate time is highly objectionable.
It has been known that small amounts of unsaturated ketones, e.g. methyl vinyl ketone and methyl isopropenyl ketone are present in the acid products and are the cause of the permanganate time reductions which have been found. This discovery is described in U.S. Pat. No. 3,347,756. As stated in this patent, the unsaturated ketones are capable of reacting reversibly with acids to form addition products which, unlike the parent ketones, are saturated and higher boiling than the C.sub.1 -C.sub.4 acids. The presence of the addition product between the unsaturated ketones and the acids in the final acid products has no effect on the permanganate time. The failure of the recovered acids to pass the test for permanganate time, however, is due to the presence of traces of the unsaturated ketones, in spite of one or more fractional distillation steps.
The rate of cracking of the addition products and of recombination of methyl vinyl ketone and acids depends upon temperature, the acid present and the concentrations of methyl vinyl ketone, acids and addition products. Formic acid reacts faster than acetic acid. Substantial recombination of methyl vinyl ketone and formic acid will occur even in 5 minutes at 90.degree. C. when excess formic acid is present.
It is now known that the unsaturated carbonyl compounds such as methyl vinyl ketone and methyl isopropenyl ketone can react with the acids in a number of ways such as condensation, polymerization and addition reactions. One particular type of addition reaction, Michael addition, involves the addition of an organic acid or something similar across the carbon-carbon double bond of the unsaturated ketone. The reaction is acid or base catalyzed and is reversible. In the case of methyl vinyl ketone and acetic acid, the reactants are much lower boiling than the product of 4-acetoxy-2-butanone (80 .degree. C. and 118.degree. C. verses 190.degree. C.). The differences in boiling temperature, along with the reversibility of the Michael addition, dictates that the equilibrium between product and reactants can be shifted by temperature. For example, if the temperature is greater than the boiling point of the methyl vinyl ketone, the methyl vinyl ketone would tend to be preferentially removed from the system as a vapor. To maintain equilibrium, some 4-acetoxy-2-butanone will revert back to methyl vinyl ketone and acetic acid. Accordingly, as long as methyl vinyl ketone or acetic acid are removed from the system, the 4-acetoxy-2-butanone will revert back to the reactants.
In previously mentioned U.S. Pat. No. 3,347,756 there is described a process for the removal of methyl vinyl ketone addition products from the acetic acid-containing LPO reaction mixture by a heat treatment process at such a temperature to shift the equilibrium and to substantially decompose the addition product into methyl vinyl ketone and the product acid. The removal may be effected by distilling the mixture to take off acetic acid and methyl vinyl ketone produced by cracking the addition product and leaving the uncracked portion of the addition product in the residue. This process, however, simply leaves a very valuable residue which is typically disposed of and still contains acetic acid product. U.S. Pat. No. 3,347,756 proposed that the separation of the liberated methyl vinyl ketone from the acetic acid-containing distillate may be effected by a variety of methods including fractional distillation or hydrogenation into a compound such as methyl ethyl ketone or secondary butanol which do not condense with acetic acid readily and which may subsequently be removed from the acetic acid. These separation techniques are difficult since it is not easy to remove the minor amounts of methyl vinyl ketone from the acetic acid by distillation. Secondly, the reaction with hydrogen does not convert all the methyl vinyl ketone and some remains to degrade the acid products. Thus, while the process disclosed in U.S. Pat. No. 3,347,756 has been useful, it is not sufficient to completely alleviate the degradation of permanganate time.
It has been found that the unsaturated ketones such as the methyl vinyl ketone and methyl isopropenyl ketone are formed during the LPO reaction and react with the acid products including acetic acid by Michael addition in the LPO reactor. The Michael adducts breakdown thermally to the unsaturated and acid components slowly such that the adducts are carried along with the acid products downstream during purification. This causes serious quality problems not only in the acetic acid but in ethyl acetate, formic acid, propionic acid, butyric acid and acetic anhydride purification. While eventually the adducts are removed as bottoms from the acid product, this results in two disadvantages. For one, there is always some reverse reaction which means that the unsaturated ketones are present in the final acid product. Secondly, the adducts contain a substantial amount of tied up acids and if these bottoms are burned, this results in an unnecessary waste.
Accordingly, it is an object of the present invention to improve the quality of acid products formed during the liquid phase oxidation of saturated aliphatic hydrocarbons.
Another object of the invention is to provide for the removal of unsaturated compounds from LPO acids which degrade the permanganate time of the acids.
Still another object of the invention is to provide a process for purifying LPO acids by breaking down the adducts which form between the acids and unsaturated ketone compounds during the LPO reaction in the initial stages of purification so that the unsaturated compounds can be removed prior to final acid product purification.
Still yet another object of the invention is to enhance the breakdown of Michael adducts formed by the addition reaction of acids to unsaturated ketones produced during liquid phase oxidation of saturated aliphatic hydrocarbons so that the unwanted unsaturated ketones can be removed form the acid products during the early stages of purification and additional acids can be freed from the Michael adducts.